function mf(url, suc, err){
    setTimeout(()=>{
        console.log(`${url}`)
        suc("successful")
    },2000)
}

mf('./a',(data)=>{
    console.log(data)
    mf('./b',(data2)=>{
        console.log(data2)
        mf('./c',(data3)=>{},()=>{})
    },()=>{})
},()=>{})